Comment 2 for bug 1078549

NB - I think this is the same issues as decribed in here:

http://docs.openstack.org/trunk/openstack-compute/admin/content/network-troubleshooting.html

        </simplesect>
        <simplesect><title>KVM: Network connectivity works initially, then fails</title>
        <para>Some administrators have observed an issue with the KVM hypervisor where instances
                running Ubuntu 12.04 will sometimes lose network connectivity after functioning
                properly for a period of time. Some users have reported success with loading the
                vhost_net kernel module as a workaround for this issue (see <link
                    xlink:href="https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/997978/">bug
                    #997978</link>) . This kernel module may also <link
                    xlink:href="http://www.linux-kvm.org/page/VhostNet">improve network performance
                    on KVM</link>. To load the kernel module, as
                root:<screen><prompt>#</prompt> <userinput>modprobe vhost_net</userinput></screen></para>
            <para>Note that loading the module will have no effect on instances that are already
                running.</para></simplesect>
    </section>

and I think this was fixed in grizzly - marking fix released